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Chapter 4. Electrical Conductivity Methods for Measuring and Mapping Soil Salinity

Chapter 4. Electrical Conductivity Methods for Measuring and Mapping Soil Salinity

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202



J. D. RHOADES



depleted and salinized by their consumption for irrigation and/or by the

return to them of salt-laden drainage water. Sufficient leaching and drainage are required to keep salinity within irrigated soils from exceeding

tolerable levels, if crop production and profitability of irrigated agriculture

are to be sustained. However, it is these very processes that often lead to the

pollution of our water resources. Currently, programs are being implemented to reduce leaching and restrictions are being legislated to control

the discharge of saline drainage water from irrigation projects. Concomitantly, reuse of saline drainage water is increasing as disposal is being

limited and as the availability of fresh water supplies is decreasing. With

less leaching and drainage and greater use of saline water for irrigation, the

soil salinity hazard potential increases.

The proper management of irrigated agriculture, especially under the

above-described conditions, requires periodic information on the soil salinity status of fields, farms, projects, and hydrogeologic areas. Only with

this information can the appropriateness and effectiveness of farm practices, of land use plans, of water quality plans, and of irrigation project

operations be assessed with respect to leaching/drainage adequacy, salt

balance, irrigation sustainability, water use efficiency, and environmental

protection. Practical methods for measuring, monitoring, and mapping

soil salinity are essential to meet these burgeoning needs. In addition,

practical procedures are needed for locating representative measurement/

monitoring sites in order to map the distribution and extent of salt-affected

soils, to delineate areas of under- and overirrigation and areal sources of

salt loading, and to monitor and assess salinity trends.

Ideally, it would be desirable to know the concentrations of the individual solutes in the soil water over the entire range of field water contents and

to obtain this information immediately in the field. Practical methods are

not available at present to permit such determinations, although determinations of total solute concentration (i.e., salinity) can be made in situ

using electrical or electromagnetic signals from appropriate sensors. Such

immediate determinations are so valuable for salinity diagnosis, inventorying, monitoring, and irrigation management needs that, in many cases,

they supplant the need for soil sampling and laboratory analyses. However,

if knowledge of a particular solute(s)concentration is needed (such as when

soil sodicity or a specificion toxicity is to be assessed), then either a sample

of soil, or of the soil water, is required to be analyzed. Of course, the

methods to accomplish this require much more time, expense, and effort

than do the instrumental field methods. Thus, a combination of the

various methods should be used to minimize the need for sample collection and chemical analyses, especially when monitoring solute changes

with time and characterizing the salinity conditions of extensive areas.



MEASURING AND MAPPING SOIL SALINITY



20 3



Assessing soil salinity is complicated by its spatially variable nature.

Numerous samples (measurements) are needed to characterize just one

field. Furthermore, soil salinity is dynamic in nature due to the influences

of varying soil/crop/irrigation management practices, water table depth,

soil permeability, evaporation and transpiration rates, rainfall amount and

distribution, and salinity of the perched groundwater. Thus, soil salinity

information needs to be updated as conditions change. When the need for

repeated measurements and extensive sampling requirements is met, the

expenditure of time and effort to characterize and monitor the salinity

condition of a large area with conventional soil sampling and laboratory

analysis procedures becomes impractical. However, rapid instrumental

field techniques for measuring soil electrical conductivity, for inferring

salinity from it, and for locating spatial position on the landscape, coupled

with use of data logging equipment, statistics, and computer-assisted mapping techniques, offer us the potential to meet our soil salinity assessment

needs in this regard. The additional use of geographic information systems

and remote sensing technology further increases this potential.

Soil salinity has been customarily defined and assessed in terms of

laboratory measurements of the electrical conductivity of the extract of a

saturated paste of a soil sample (EC,), because electrical conductivity is a

practical index of the total concentration of ionized solutes in an aqueous

sample and the saturation percentage (SP) is the lowest water/soil ratio for

the practical laboratory extraction of readily dissolvable salts in soils (U.S.

Salinity Laboratory Staff, 1954). But it can also be determined from the

measurement of the electrical conductivity of a soil water sample (EC,).

This latter measurement can be made either in the laboratory on a collected sample or directly in the field using in situ, imbibition-type salinity

sensors. Alternatively, salinity can be indirectly determined from measurement of the electrical conductivity of a saturated soil paste (EC,) or of the

electrical conductivity of the bulk soil (EC,). EC, can be measured either

in the laboratory or field using simple and inexpensive equipment. EC, can

be measured in the field, using electrical-type probes placed in contact with

the soil, or remotely, using electromagnetic induction devices. The latter

two measurements require more expensive, but very cost effective, equipment. From EC, and EC,, soil salinity can be derived in terms of either

EC, or EC, . The appropriate method to use depends on the purpose of the

determination, the size of the area being evaluated, the number and frequency of measurements needed, the accuracy required, and the available

equipment/manpower.

This paper reviews the various electrical conductivity methods for measuring soil salinity together with compatible ways for mapping it, including

establishing the locations of measurement sites. Advantages and limita-



204



J. D. RHOADES



tions of the alternative methods are discussed and a practical integrated

mobile system for measurement/monitoring/mappingis described. For

earlier treatises on the instrumental field methodology of soil salinity

measurement and assessment, see Rhoades (1976, 1978, 1984, 1990a,b,

1992a,b), Rhoades and Oster (1986), Rhoades and Corwin (1984, 1990),

and Corwin and Rhoades ( 1990).



11. DETERMINATION OF SOIL SALINITY FROM

AQUEOUS ELECTRICAL CONDUCTIVITY

A. PRINCIPLES

OF AQUEOUS

ELECTRICAL

CONDUCTMTY

Electrical conductivity is a numerical expression of the inherent ability

of a medium to cany an electric current. Because the EC of an aqueous

solution is closely related to the total concentration of dissolved electrolytes (ionic solutes) in the solution (water itself is a very poor conductor of

electricity), it is commonly used as an expression of the total dissolved salt

concentration of an aqueous sample, even though it is also affected by the

temperature of the sample and by the mobilities, valences, and relative

concentrations of the individual ions comprising the solution. Furthermore, not all dissolved solutes exist as charged species; some combine to

form ion pairs, and some of the ion pairs are neutral and do not contribute

to electrical conductivity.

The determination of EC generally involves the physical measurement

of the resistance (R),expressed in ohms, of a material. The resistance of a

conducting material (such as a saline solution) is inversely proportional to

its cross-sectional area (A) and directly proportional to its length (L). The

magnitude of the resistance measured therefore depends on the characteristics (dimensions) of the conductivity cell used to contain the sample and

the electrodes. Specific resistance ( R , ) is the resistance of a cube of the

sample 1 cm on edge. Practical cells are not of this dimension and measure

only a given fraction of the specific resistance; this fraction is the cell

constant ( K = R/R,).

The reciprocal of resistance is conductance (C). It is expressed in reciprocal ohms, i.e., mhos. When the cell constant is applied, the measured

conductance is converted to specific conductance (i.e., the reciprocal of the

specific resistance) at the temperature of measurement. Herein, specific

conductance is called electrical conductivity, EC:

EC = l/R,



=K/R



(1)



Electrical conductivity has been customarily reported in micromhos per



MEASURING AND MAPPING SOIL SALINITY



20s



centimeter (pmho/cm), or in millimhos per centimeter (mmho/cm). In the

International System of Units (SI), the reciprocal of the ohm is the siemen

(S) and, in this system, electrical conductivity is reported as siemens per

meter (S/m), or as decisiemens per meter (dS/m). One dS/m is equivalent

to one millimho/cm.

Electrolytic conductivity (unlike metallic conductivity) increases with

temperature at a rate of approximately 1.9%/1"C.Therefore, EC needs to

be expressed at a reference temperature for purposes of salinity expression;

25 "Cis most commonly used in this regard. The best way to correct for the

temperature effect on conductivity is to maintain the temperature of the

sample and cell at 25 f 0.5"Cwhile EC is being measured. The next best

way is to make multiple determinations of sample EC at various temperatures both above and below 25"C, then to plot these readings and interpolate the EC at 25°C from the smoothed curve drawn through the datapairs.

For practical purposes of agricultural salinity appraisal, EC can be measured at one known temperature other than 25 "Cand then adjusted to this

latter reference using an appropriate temperature coefficient (A). These

coefficients are usually based on sodium chloride solutions, because their

temperature coefficients closely approximate those of most surface waters

and groundwaters. Potassium chloride solutions are not generally used for

this purpose because they have a lower temperature coefficient of conductivity than is typical of most natural waters or soil extracts. Another

limitation in the use of temperature coefficients to adjust EC readings to

25°C is that they vary somewhat with solute concentration. The lower the

concentration, the higher the coefficient, due to the effect that temperature

has on the dissociation of water. However, for practical needs, these limitations may be ignored and the value off; may be assumed to be single

valued. It may be estimated as follows:



+



f ; = 1 0.019(t - 25)



(2)



or



f ; = (0.0004)t' - (0.0430)t



+ 1.8 149



(3)

The latter relation was derived from the data given in Table 15 of Handbook 60 (U.S. Salinity Laboratory Staff, 1954). In turn, the EC at 25°C

(EC25) is estimated by multiplying the EC measured at temperature t (EC,)

by the temperature coefficient as follows:

EC25 = EC,f;



(4)



Because of differences in the equivalent weights, equivalent conductivities, and variations in the proportions of the various solutes found in soil

extracts and water samples, the relationships between EC and total solute

concentration and osmotic potential are only approximate. They are still



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